The generation and regulation of cardiac function involves a sequence of changing molecular interactions within and between all of the muscle protein subunits. We propose to study the details of these changes within tropomyosin, within actin and between the actin-tropomyosin complex with in vitro studies utilizing fluorescence probes located in certain specific places on the surface of these proteins. With measurements of fluorescence lifetime and fluorescence polarization on certain environmentally sensitive probes and suitable donor-acceptor energy transfer probes, changes in tropomyosin subunit conformation, changes in actin subunit conformation and tropomyosin movement on the thin filament will be monitored. Studies of tropomyosin conformational changes associated with regulation will also be studied with chemical modification techniques which emphasize SH groups. The conformational consequences of the heterogeneity of composition of cardiac tropomyosin from different animals will also be studied. The broad objective is to provide a molecular basis for the understanding of cardiac regulation.